The present invention relates generally to a rotating blade for a steam turbine and more particularly to a rotating blade with geometry capable of increased operating speeds for use in a latter stage of a low pressure section of a steam turbine.
The steam flow path of a steam turbine is generally formed by a stationary casing and a rotor. In this configuration, a number of stationary vanes are attached to the casing in a circumferential array and extend inward into the steam flow path. Similarly, a number of rotating blades are attached to the rotor in a circumferential array and extend outward into the steam flow path. The stationary vanes and rotating blades are arranged in alternating rows so that a row of vanes and the immediately downstream row of blades form a stage. The vanes serve to direct the flow of steam so that it enters the downstream row of blades at the correct angle. Airfoils of the blades extract energy from the steam, thereby developing the power necessary to drive the rotor and the load attached thereto.
As the steam flows through the steam turbine, its pressure drops through each succeeding stage until the desired discharge pressure is achieved. Thus, steam properties such as temperature, pressure, velocity and moisture content vary from row to row as the steam expands through the flow path. Consequently, each blade row employs blades having an airfoil shape that is optimized for the steam conditions associated with that row.
In addition to steam conditions, the blades are also designed to take into account centrifugal loads that are experienced during operation. In particular, high centrifugal loads are placed on the blades due to the high rotational speed of the rotor which in turn stress the blades. Reducing stress concentrations on the blades is a design challenge, especially in latter rows of blades of a low pressure section of a steam turbine where the blades are larger and weigh more due to the large size and are subject to stress corrosion due to moisture in the steam flow.
This challenge associated with designing rotating blades for the low pressure section of the turbine is exacerbated by the fact that the airfoil shape of the blades generally determines the forces imposed on the blades, the mechanical strength of the blades, the resonant frequencies of the blades, and the thermodynamic performance of the blades. These considerations impose constraints on the choice of the airfoil shape of the blades. Therefore, the optimum airfoil shape of the blades for a given row is a matter of compromise between mechanical and aerodynamic properties associated with the shape.